The present invention relates generally to gauges for measuring the thickness of coatings and more specifically relates to gauges for magnetically measuring the thickness of a nonmagnetic coating on a magnetizable material.
Various magnetic coating thickness gauges are known in the art wherein a probe surface is magnetically maintained in contact with a coating to be measured. Generally, a spring biases the probe surface away from the coating to be measured with the force applied by the spring then gradually increased by an operator until the probe surface separates from the coating to be measured. At that instant, a pointer on a scale indicates the thickness of the coating. The scale is typically rotated to tension the spring and thereby gradually increase the spring force applied to the probe surface.
In some magnetic coating thickness gauges, a magnet is provided at one end of a balance arm with the magnet placed in direct contact with the coating to be measured. In other gauges, an intermediary material is provided between the magnet and the coating to be measured.
In the known magnetic coating thickness gauges having a balance arm, however, the magnet or the intermediary material which contacts the coating to be measured is of a relatively soft material with the result that the magnet or intermediary material frequently wears away during a relatively short period of time. Since the erosion of the magnet or the intermediary material affects the magnetic attraction between the contact surface and the material bearing the coating to be measured, the accuracy of the magnetic coating thickness gauge is seriously affected.
For example, in the measurement of chrome plating of ferrous materials, conventional magnetic coating thickness gauges oftentimes become useless or in need of calibration in a period of use as short as one day. Accordingly, the need exists for a magnetic coating thickness gauge having a coating surface of a durable material sufficient to permit prolonged use on a variety of hard coatings.
Magnetic coating thickness gauges are known which have a probe contact surface of a wear resistant material. In U.S. Pat. No. 3,999,120, of Streng, for example, a spherical sector is attached to the front faces of a soft-iron magnetic yoke, and forms the probe tip for the gauge. The spherical sector is a ground half ball-bearing which is electrically welded to a flat plate at the end of a threaded shaft. The spherical sector is provided with a titanium carbide layer by gaseous diffusion to provide a wear resistant surface. A similar arrangement is shown in U.S. Pat. No. 4,041,378 of Ott.
To provide a wear resistant surface in the manner shown by the Streng patent, however, is particularly expensive and difficult to accomplish. Accordingly, the need exists for a relatively low cost magnetic thickness gauge having a wear resistant probe tip.
In a conventional magnetic coating thickness gauge, the gauge is calibrated during assembly through the use of an electromagnetic coil which externally applies a magnetic field to the magnetic material in the probe assembly to provide the magnetic material with a desired degree of magnetization. Such a calibration procedure is particularly troublesome to accomplish and the recalibration procedure is even more troublesome to perform should the gauge require recalibration at some future time. In order to recalibrate such a magnetic probe assembly, at least a partial disassembly of the magnetic coating thickness gauge is typically required.
Since the range over which the magnetization of the magnetizable material in the probe assembly can be varied is typically rather small, the conventional magnetic coating thickness gauge requires a particularly close correlation between the biasing force provided by the spring and the magnetic attractive force provided by the probe assembly. In this way, a relatively high level of precision is necessary for the spring assembly which biases the probe assembly away from the coating to be measured with the result that either relatively expensive springs are required or that the rejection rate for springs of lesser expense is typically high.
Therefore, the need exists for a magnetic coating thickness gauge having a calibration arrangement wherein the range of attractive force provided by the magnetizable probe assembly is relatively great and easily varied so as to accommodate a correspondingly large range of biasing force provided by the spring assembly.
In a commercially available magnetic coating thickness gauge of Electro-Physik (such as is shown in U.S. Pat. Nos. 4,160,208, 3,761,804, 3,699,487 and 3,521,160 which are hereby incorporated by reference) a handle is provided rearwardly of an indicator scale with the handle normally grasped by the operator during use. Typically, the thumb of the operator is used to rotate an adjustment wheel which varies the biasing force provided by the spring. The thickness gauge is supported both adjacent a probe assembly and immediately beneath the indicator scale (at about a mid-section of the gauge) with the result that the operator typically grips the gauge behind the indicator scale. The downward force applied by the operator rearwardly of the indicator scale tends to pivot the gauge about the support provided beneath the indicator. If the gauge is pivoted, the probe will no longer contact the coating to be measured resulting in a false reading for the gauge. Accordingly, the use of two hands to operate the conventional gauge is frequently necessary with the operation of the conventional gauge oftentimes awkward to perform.
Incorrect operation of the conventional thickness gauge also typically results from an inadvertent placement of the user's hand over a protrusion of the balance arm (which is provided to enable the user to initially position the probe assembly in contact with the coating to be measured). If the user's hand obstructs the free movement of the protrusion (and in turn the balance arm), the probe assembly cannot freely lift away from the coating to be measured (when the spring force equals the magnetic attractive force) with the result that an inaccurate reading is obtained.
Accordingly, the need exists for a magnetic coating thickness gauge having a housing and balance arm assembly which facilitates the correct operation of the thickness gauge and which facilitates operation of the gauge with only one hand.